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Nightmare Fuel: Final Fantasy XIV
The Ultima Weapon's entrance. You think you've beaten Garuda, but her subjects' worship makes her unbeatable. Then Gaius appears, and taunts her. She responds by forcing the captive beast men of the other primals to pray to Ifrit and Titan, and with a push from Garuda, they manifest right then and there. All three primals you have fought are in one place. Couldn't get any worse, right? But then Ultima Weapon drops in. Ifrit attacks it first... and is defeated in seconds. And is then absorbed into Ultima Weapon. It then makes short work of Titan, absorbing him too. Garuda is the only one left. Up until now, Garuda hasn't shown anything other than contempt for everything other than herself. But faced with Ultima Weapon, she expresses pure fear for the first time. But she's unable to stop the inevitable; once Ultima Weapon gets a hold of her, it crushes her head, absorbing the third primal. This thing has the power of three primals, and is in Garlean hands.
Tempering. Not only can a primal convert someone to their side in a burst of power — with no way to prevent, dodge, or block it, unless you have the Echo — there is no known way to reverse the process. Tempered victims are actually routinely killed off by the good guys as a result of this, because they view it as a Fate Worse than Death and leaving them alive only serves to potentially strengthen the primal that has them under control. Worse, while tempering immediately twists loyalty to the primal, Color-Coded for Your Convenience isn't necessarily enforced. Tempered sylphs are generally identifiable by their purple coloring instead of green (although they still have the power to shapeshift so even that isn't a certain thing), but we see friendly Amalj'aa with red clothing that help new Black Mages, and tempered soldiers wearing the clothes of their country as they commit treason. The Beastmen tribe and Summoner quests only augment the horror here : *anyone* can be tempered or claim someone else to be tempered.
The hedge sculptures in Haukke Manor's courtyard. Looming humanoid figures with holes through their heads — a reminder that the lady of the house, Elizabeth Bathory by any other name, has been murdering beautiful young women and shredding their faces beyond recognition.
A great deal of The Lost City of Amdapor certainly qualifies. For anyone familiar with Nausicaa, the ruins in this dungeon bear a striking similarity to the forgotten kingdoms grown over by the toxic jungle, and you can plainly see the horrifying corruption spread far into the distance - what isn't obscured by clouds of choking spores, at any rate. Worse yet is the initial boss, a half-decayed, still-living goobue that routinely devours your fellow party members. If you don't kill it fast enough, the lifeless corpses of your comrades will be regurgitated onto the pock-marked stone flooring. And then you get to the interior of Amdapor, seeing the marvelous architecture that once made up the whole city, and discover Diabolos at the end...and then that music starts playing...
The previews for Tam-Tara Deepcroft (Hard) showed the final boss arena to be littered with pentagrams, runes, and the name "Avere" repeated over and over again, in Eorzean script. Avere was the name of the tank of an NPC party who shows up during the initial, main-story Tam-Tara questline. He died because his fiancee, Edda, a Conjurer, couldn't keep up on heals. She originally kept his severed head so that she could give him a burial... but, all signs point to Edda trying to resurrect him using Blood Magic.
Later previews showed Edda with a giant, terrifying, demonic looking head that has vaguely Ahriman-like features, except that its arms are its nerve endings.
Previews also showed Edda self-harming in order to power up said head. The animations for it are... unsettling.
Later, once you do the dungeon (and are in a party willing to read the 'Torn Folio' lore items), Edda specifically references the player character, giving an added touch of skin-crawling regret for earlier sympathies, and shades of My God, What Have I Done?
The ending of the quest, "Corpse Groom": After you defeat the Avere-head, Edda accidentally falls from the platform to her death. You go outside to assure her old teammate that it's all over — and he looks across the chamber to see Edda standing there looking at him with the creepiest Slasher Smile. He promptly flees, you turn around — and she's not there. Sure, Paiyo's exaggerated facial reaction's kinda funny, but still...